Research paperAssessment of restoration success of former metal mining areas after 30 years in a highly polluted Mediterranean mining area: Cartagena-La Unión

The first ecological restoration of mining tailing were carried out in the Mining District of Cartagena-La Unión in 1982, by means of the sealing of the mining wastes with a layer of 0.5 m of soil, to allow the colonization by the vegetation. This study provides an evaluation of the metal mobilization on those mining ponds restored 30 years ago under semiarid Mediterranean climate conditions. With this objective, to guarantee independence of samples, up to six restored mine tailings with a well-developed vegetation cover with trees and herbaceous species were studied. The metals content in the soil profile was evaluated, as well as the heavy metals transference to wild plant communities. Results show that after 30 years the sealing did not prevent the ascent of heavy metals to the soil surface, reaching levels which are above the levels established by normative. The lowest values in the soil profile were found in the soil surface but even those, the total heavy metal showed 12 mg/kg, 4616 mg/kg, 67 mg/kg, and 3635 mg/kg, for Cd, Pb, As and Zn respectively in topsoil. Furthermore Dittrichia viscosa, Tetraclinis articulata, Thymelaea hirsuta, Teucrium capitatum and Hyparrenia hirta showed excessive levels in its tissues for Cd or Zn. Therefore topsoil of restored mining tailings does not remain unpolluted after 30 years and still remain a high mobility and bioaccumulation for metals, especially for Cd and Zn. In this regard, the sealing of the mining tailing with a soil cover is not enough and before the sealing the installation of physical barriers on mining waste should be employed to breaks the bottom-up flux of metallic salts. The efforts should be oriented to look for cheaper but effective materials to be employed in the building of capillary barriers which can be used in large areas. It should be considered a basic step for environmental restoration of these mining areas in order to avoid capillary rise of heavy metal salts that finally will contaminate again the soil surface in the long term.